The present invention relates generally to methods for improving adhesion between substrates and polymers and products made according to such methods. Such methods may produce products in areas as diverse as industrial composite hoses and medical garments.
Substrate composite articles having a plurality of layers, especially those used to make garments, air bags and industrial composites, could be vastly improved if there was a method for improving adhesion between substrate and polymer layers; while maintaining the malleability and characteristics of the original substrate. For example, a problem that has long plagued the art has been the inability to construct a long-lasting, durable, reusable medical garment that is breathable and comfortable but impermeable to disease causing microorganisms such as viruses and bacteria. Moreover, industrial composite articles with increased durability, malleability and strength have been sought after by industry leaders for many years. What is needed is a single method of improving adhesion between substrate and polymer layers without adversely affecting desired properties of the original untreated substrate.
Methods for improving adhesion between multiple layers typically involve a combination of surface modification techniques. Several different physical types of surface modification exist. One type of surface modification is to covalently bind a modifier to a surface of a substrate material. Such binding can be achieved in many different ways such as chemical grafting onto the surface of the substrate through condensation or high energy addition reactions, or oxidizing the substrate away leaving a covalently bound modified surface. Covalently bound modifiers are usually the most durable surface modifications. However, such techniques are complicated, expensive and often environmentally hazardous to employ. Another type of surface modification is to cause an association or entrapment of the modifying molecule (or part of the molecule) with the substrate material. This commingling of modifier molecules and substrate relies on molecular attractions such as Van der Waals forces, dipole/dipole interactions, Hydrogen bonding, as well as steric factors to hold the modifier in/on the surface of the substrate. The factors that effect this type of reaction are similar to those that effect thermosol dyeing or blooming. Still another type of surface modification involves the retention of modifier by substrate with only adhesive and cohesive forces between the modifier to the substrate and the modifier to itself respectively. This last technique is the most common type of surface modification in the textile area and is the one in which the present invention makes a dramatic difference in the field of adhesion.
Methods for improving adhesion between substrate and polymer layers have previously come at the expense of other qualities such as durability, malleability, the environment, or performance characteristics. Addressing one aspect of desired qualities usually results in sacrificing other qualities. Conventional treatments for improving adhesion between substrate and polymer layers are typically unable to solve this dilemma and fall into the general categories of (i) surface coatings; (ii) saturations or impregnations; (iii) layers of fibers and/or polymers; (iv) unique chemical compositions; and (v) combinations of the foregoing.
I. Coatings
Coatings can be one or two sided but tend to be step gradients from one surface through the width of the substrate being treated, as opposed to homogeneous materials or continuous gradients. A step gradient has certain intrinsic disadvantages, due mostly to the fact that the coating composition contacts the substrate at one surface, thereby causing a substrate/coating-composition interface. Adhesion at this substrate/coating-composition interface derives mostly from surface forces, less than optimal mechanical interlocking, and sometimes little to no contribution from the cohesive strength of the modifying or coating material. Secondly because of the disparate materials plied together the resultant tactile properties of the composite (i.e.xe2x80x94hand, drape, etc.) are usually distinctly different than the base fabric. Typically, this interface tends to separate upon prolonged washing conditions or upon high stress conditions.
Prior fluorochemical and silicone (See U.S. Pat. Nos. 3,436,366; 3,639,155; 4,472,470; 4,500,584; and 4,666,765) fabric coating treatments evidently can protect only that side of the fabric upon which they are disposed. Such treatments significantly alter the hand, or tactile feel, of the treated side. Prior silicone fabric coatings typically degrade the tactile finish, or hand, of the fabric and give the coated fabric side a rubberized finish which is not appealing for many fabric uses, particularly garments. Coating techniques also encounter durability issues.
Porous webs have been further shown to be surface coated in, for example, U.S. Pat. Nos. 4,478,895; 4,112,179; 4,297,265; 2,893,962; 4,504,549; 3,360,394; 4,293,611; 4,472,470; and 4,666,765. These surface coatings impart various characteristics to the surface of a web, but remain on the surface and do not provide a film over the individual internal fibers and/or yarn bundles of the web. In addition, such coatings on the web surface tend to wash away quickly.
II. Saturation and Impregnation
Prior treatments of webs by saturation or impregnation with a polymer material, such as a silicone resin, polyurethane or neoprene material, are typically accomplished by immersion, using a low viscosity liquid so that the low viscosity liquid can flow readily into the web, and be adsorbed or absorbed therewithin. Immersion applications of one hundred percent (100%) solids, solvent dissolved solids, or aqueous emulsions can be performed by running a fabric through a bath and then drying. Particularly for flexible webs, including fabrics, an immersion application of a liquid or paste composition to the web is achieved, for example, by the so-called padding process wherein a fabric material is passed first through a bath and subsequently through squeeze rollers in the process sometimes called single-dip, single-nip padding. Alternatively, for example, the fabric can be passed between squeeze rollers, the bottom one of which carries the liquid or paste composition in a process sometimes called double-dip or double-nip padding. Usually, one hundred percent (100%) solids applications rely on low molecular weight materials (with viscosity""s low enough for processing) that tend to yield a treated substrate with poor mechanical properties or higher molecular weight materials which often do not yield optimal penetration into the substrate. Solvent processing has environmental and economic issues such as removal of the solvent, fate of Volatile Organic Compounds (VOCs), and government permit requirements which are becoming stricter. Solvent and emulsion processing both are thermodynamically driven to yield low surface coverage due to poor substrate wet out (greater than zero contact angle), if the modifier is of a lower surface tension than the substrate (e.g. Durable Water Repellantxe2x80x94DWR). Provided the surface of the substrate is wet out by the modifier these techniques still require removal of the solvent or aqueous media which tends to cause imperfections in the polymer network, manifested as reduced mechanical properties of the modifier/substrate combination.
The silicone resin treated product is typically a rubberized web, or fabric, that is very heavily impregnated with silicone. For example, U.S. Pat. No. 2,673,823 teaches impregnating a polymer into the interstices of a fabric and thus fully filling the interstices. Thus, this patent provides no control of the saturation of the fabric and instead teaches full saturation of the interstices of the fabric. Such a treated web is substantially devoid of its original tactile and visual properties, and instead has the characteristic rubbery properties of a cured silicone polymer.
Prior treatments of webs that force a composition into the spaces of the web while maintaining some breathability have relied on using low viscosity compositions or solvents to aid in the flow of the composition. U.S. Pat. No. 3,594,213 describes a process for impregnating or coating fabrics with liquified compositions to create a breathable fabric. Thus, the method of this patent imparts no energy into the composition to liquify it while forcing it into the spaces of the web, because the composition is substantially liquified before placement onto and into the web. U.S. Pat. No. 4,588,614 teaches a method for incorporating an active agent into a porous substrate. This process utilizes a solvent to aid in the incorporation of the active agent into the web.
Still other impregnation techniques employ dispersed polymer compositions in some type of solvent, such as water or volatile organic based solvents. The placement of polymer upon individual elements of the web is random and inconsistent. The polymer fails to completely encase the individual elements or fibers of the web, thus creating individual spots of polymer. Each spot of polymer has a polymer/fiber interface whereby adhesion results. Such techniques are illustrated in different capacities under U.S. Pat. Nos. 5,128,198 and 5,371,136. The adhesive strength of the resultant product is limited by the bond strength of the polymer particles to the individual fibers or elements of the web.
III. Layers
Several references describe laminates or layers of fabrics and/or polymers. Laminations use an adhesive tie coat to keep a film in contact with the fabric surface. This technique exhibits the same limitations described above for coatings, as well as environmental issues with the adhesives and any other part of the film preparation process. Additional difficulties are encountered in ensuring that the mechanical performance differential between the substrate, adhesive, and film is balanced. For example, if shrinkage of any of the three materials passes the initial yield stress of either of the other materials there will be deformation, and if it passes the ultimate tensile strength there will be delamination of the composite. U.S. Pat. Nos. 4,872,220; 5,024,594; 5,180,585; 5,335,372; and 5,391,423; describe articles that use layers of fabrics and/or polymers to protect blood, microbes, and viruses from penetrating through the fabrics. Similarly, U.S. Pat. No. 4,991,232 describes a medical garment comprising a plurality of plies to prevent blood from penetrating through the garment. Layers of fabrics and/or polymers traditionally result in heavier garments and utilize additional raw materials. Moreover, the coating of a polymer upon a web which has been treated by the above techniques, exhibits the same limitations as discussed above.
Some layering techniques, particularly related to industrial composites, require a combination of steps to improve adhesion and/or obtain multiple layers of polymer and fabrics while shaping the article. U.S. Pat. No. 3,762,978 describes a process for preparing a surface of a cured silicone polymer with a mineral acid for the purpose of adhering another uncured silicone polymer composition. This technique requires specific compositions and is not universally available for all compositions. Moreover, the technique relies on an initial silicone polymer coating, having the limitations of coatings discussed above.
IV. Chemical Compositions
Most industry efforts to improve adhesion of one layer to another have focused primarily on the chemistry involved in adhesion. Many patents, too numerous to cite, involve unique chemical compositions. The large number of patents directed to this art are testimony to the fact that each composition has unique abilities and limitations related to adhesion, durability and/or other performance characteristics. A sampling of patents related to unique compositions for improved adhesion comprise U.S. Pat. Nos. 4,681,808; 5,292,586; 5,360,852; 5,416,144; 5,374,485; 5,342,870; 4,525,400; 4,483,973; 5,308,887; 5,190,827; 5,175,058; 5,175,057; 5,128,394; 5,096,981; 5,028,485; 4,988,779; 4,794,192; 5,436,303; 5,399,614; 5,714,265; 4,918,126; 4,205,559; 5,023,288; 4,942,093, 5,503,940, 5,700,532; European Publication No. 0 491 483 A1; and an article by Stein et al., in Macromolecules, 19: 2291-2294, 1986. Most of these references identify specific chemical moieties related to adhesion. Two such patents, U.S. Pat. Nos. 5,714,265 and 5,700,532 describe the use of two different curing agents, a platinum based catalyst polymer and a peroxide based catalyst polymer, interacting to create an improved adhesion.
V. Combination of Techniques
Some techniques do not easily fall within any of the categories listed above because they rely on a plurality of factors to improve adhesion of multiple substrate/polymer layers. These techniques typically recognize that mechanical interlocking and chemistry play a role in the adhesion process. However, the methods employed to achieve such mechanical interlocking of one layer to another are limited to traditional methods described above, such as impregnation or emulsion/immersion techniques, exhibiting the limitations of such techniques as described above.
U.S. Pat. No. 3,962,511 describes a textile fabric composite prepared by the sequential steps of (A) encapsulating the fibers of the fabric with a polyurethane reaction mixture, (B) drying and at least partially curing said reaction mixture, (C) adhering an overlay of a flexible thermoplastic to one surface of said encapsulated fabric and (D) pressing said thermoplastic into the interstices of said encapsulated fabric under pressure and with the application of heat. Although the term xe2x80x9cencapsulatexe2x80x9d is used, the method of achieving such xe2x80x9cencapsulationxe2x80x9d is nothing more than a standard organic solvent immersion technique, as described above. This technique is environmentally hazardous and produces random, discontinuous placement of polymer upon the surfaces of fibers, resulting in fiber/polyurethane interfaces. Since the polyurethane does not truly encapsulate (i.e. surround) the individual fibers, the adhesion is limited to the fiber/polyurethane bond. Moreover, the partial curing of the polyurethane before application of another layer results in a less malleable textile substrate than the original textile web, thereby creating problems for shaping the treated substrate for composite articles, such as hoses or belts.
In an effort to address the malleability/flexibility concerns of other techniques, U.S. Pat. No. 4,109,543 describes a composite laminate process comprising a hot melt type thermoplastic material which is heated to allow it to penetrate into the base textile substrate. The thermoplastic is heated to penetrate into the interstices of the fabric a maximum of from about 15% to about 75%, but does not penetrate a substantial amount into the yarn structure itself, thereby permitting an optimum amount of flexibility of the laminate. This xe2x80x9chot meltxe2x80x9d technique improves the flexibility of the structure but is limited by the adhesion of the thermoplastic to the textile substrate. Moreover, the thermoplastic material does not surround the individual elements or fibers in the base textile substrate, since it does not penetrate into the yarn structures. U.S. Pat. No. 4,181,157 addressed the malleability/flexibility concern by cutting the woven glass fiber substrate into strips to increase the flexibility of the normally coiled sheet when installed on the line to permit the sleeve and protected line to assume curved configurations without buckling or separation of the sleeve. This technique creates a plurality of seams that create multiple points for potential leakage.
Still other techniques rely on pressuring a thermoplastic material into a base substrate or wrapping all the substrate and polymer layers into a predetermined shape and curing. These techniques rely on standard coating or pressuring techniques to create adhesion between layers of polymer and substrates. Such techniques are further described in U.S. Pat. Nos. 3,969,177; 3,972,757; 4,511,615; 5,398,729; and 5,677,046. These techniques contain some of the same limitations discussed in the above categories.
FIGS. 1a, 1b and 1c illustrate three types of prior art treatments discussed above. FIG. 1a illustrates a cross section of an immersion treatment of a fibrous textile substrate. The particulate spots on the surface of the fibers represent polymer residuals. FIG. 1b illustrates a cross section of a coating treatment of a fibrous textile substrate. The coating interacts with the fibers along one surface, creating a coating/fiber interface. FIG. 1c illustrates a cross section of a lamination treatment of a fibrous textile substrate. The laminate is adhered to one surface with an adhering agent, creating a laminate/fiber interface.
What is needed in the art is a single method for improving both adhesion between multiple layers and durability of such a composite article, while maintaining the malleability and characteristics of the base substrate.
The present invention relates to a method for improving both adhesion between substrate and polymer layers and the durability of such composite structures; while maintaining the malleability and preferred characteristics of the original substrate. The method comprises placing an uncured, substantially solvent free, polymer composition exhibiting thixotropic or pseudoplastic characteristics, onto a base substrate having structural elements and interstices therebetween; shear thinning the polymer composition to place it into the substrate, thereby encapsulating most of the structural elements while leaving a plurality of interstitial spaces open; applying a layer or film of polymer or thermoplastic material to at least one surface of the uncured encapsulated base substrate; pressuring the layer into the interstices of the uncured encapsulated base substrate to form a chemical and mechanical interlocking bond; and curing the completed composites. Optionally, the multi-layer composite may be formed into a shape prior to curing. This method produces multi layer composite articles and medical garments exhibiting increased adhesion and durability with excellent malleability and flexibility over conventional methods, as described herein below.
The methods employed for shear thinning one hundred percent (100%) solid polymers or substantially solvent free polymer compositions are described in the following patents and patent applications, all of which are incorporated herein by reference in their entirety, including any drawings: pending U.S. patent application Ser. No. 09/014,316 filed on Jan. 27, 1998; pending U.S. patent application Ser. Nos. 08/962,700, 08/963,636, 08/962,698 and 08/963,663, all filed Nov. 3, 1997; pending U.S. patent application Ser. No. 08/472,568 filed on Jun. 7, 1995; allowed U.S. patent application Ser. No. 08/442,983 filed on May 17, 1995, allowed Feb. 18, 1998; allowed U.S. patent application Ser. No. 08/407,191 filed on Mar. 17, 1995, allowed Oct. 21, 1997; U.S. Pat. No. 5,698,303, issued Dec. 16, 1997; U.S. Pat. No. 5,418,051 issued May 23, 1995; U.S. Pat. No. 5,209,965 issued May 11, 1993; U.S. Pat. No. 5,004,643, issued Apr. 2, 1991; and U.S. patent application Ser. Nos. 167,630, 167,643, 167,797 and 167,869 all filed on Mar. 14, 1988, now abandoned. Manipulation and alteration of the polymer composition and the web according to the methods of the above incorporated patents and patent applications, produces a web that either: (1) has a plurality of its structural elements encapsulated by the polymer composition while at least some of the interstitial spaces of the web are open; or (2) has an internal layer extending through the web; or (3) has both encapsulated structural elements and an internal layer of polymer composition.
The method employed for precision placement of thin polymeric films within substrates to achieve improved substrate performance is conducted substantially without the use of solvents. A polymeric composition is applied onto the surface of a web by a variety of means. After the polymer is applied to the surface of the web, the polymer composition is preferably immediately shear thinned to controllably and significantly reduce its viscosity and place it into selected places within the web. To aid in this process, the web is preferably distorted, typically by stretching at the location of the shear thinning. This distortion facilitates the entrance of the polymer composition into the web by creating a double or dual shear thinning. In the case of the web, this is produced by the combination of the edge condition of the blade, the engineered shear thinnable polymer, the speed of the web, and the subsequent repositioning of the fibers and filaments after their immediate passage under the edge of the blade.
Controlled placement of the polymer composition within a base web or substrate may be performed by a basic embodiment of a machine in accordance with U.S. patent application Ser. No. 08/407,191, filed on Mar. 17, 1995 and allowed on Oct. 21, 1997. The technique can be as simple as an applicator to apply viscous polymer to the surface of the web, a pair of facilities for applying tension to a section of the web and a blade forced against the web in the section under tension. The web is pulled under tension past the blade, or, alternatively, the blade is moved relative to the web, and the forces generated by the blade cause the polymer composition to flow into the three-dimensional matrix of the web, and controllably be extracted out of the web leaving a thin film of polymer encapsulating selected fibers, or an internal layer of polymer, or both. Tension on the web is preferably released thereafter, and for purposes of the present invention, the web is left uncured for the subsequent application of a layer or film, as described herein.
The ability to control the placement of polymers within a base substrate correlates directly to controlling the surface area within a given volume of the base substrate that is available for bonding to a subsequently applied layer or film. The result of this control translates into control over the degree of chemical and mechanical interlocking of the subsequently applied layer or film.
A layer or film of polymer or thermoplastic material is applied to at least one surface of the uncured, encapsulated base substrate via conventional coating and/or laminating techniques. Such techniques include, but are not limited to, knife-over-air, knife-over-roll, roll coating, reverse roll coating, gap coating, extrusion coating and other techniques of similar import. In one preferred embodiment of the present invention, the layer or film is applied using a knife-over-roll apparatus and method supplied by Mascoe Systems Corporation in Mauldin, S.C.
Alternative methods for layering encapsulated base substrates and subsequent polymer or thermoplastic materials may be employed, as described herein. One key aspect of all such methods is that the applied polymer or thermoplastic material(s) is pressured into the interstices of the encapsulated base substrate to form a chemical and mechanical interlocking bond. Some combinations of layering may include, but are not limited to, (1) a single encapsulated base substrate with one layer of polymer or thermoplastic material pressured together (xe2x80x9cone plyxe2x80x9d); (b) multiple xe2x80x9cone plyxe2x80x9d layers pressured together such that the encapsulated base substrate either remains exposed to air or is sandwiched between adjacent polymer or thermoplastic layers; and (c) a plurality of encapsulated base substrates alternatively stacked between a plurality of polymer or thermoplastic materials and then pressured together. It is to be understood that other such layering combinations may be resorted to for particular applications. The pressuring can occur at any stage of layering or alternatively, at different stages of the layering of encapsulated base substrates and subsequent polymer or thermoplastic materials. One key aspect for some applications is that during this preparation of multiple layers, the encapsulated base substrate is not cured, or at most, is only partially cured, maintaining its xe2x80x9ctackyxe2x80x9d characteristic. This allows the composite article to maintain its malleability in order to be shaped into a variety of articles such as ducts, hoses and seals. Once shaped, the multi layer composite articles may be cured. Moreover, results indicate that by leaving the base substrate uncured prior to the application of a layer or film, superior performance results are obtained.
Since the base textile substrate is uncured prior to the application of a layer or film, it is possible to utilize a two part polymer composition such as a two part, platinum cured, RTV silicone polymer supplied by Dow Corning, GE Silicones, Wacker-Chemie or any other commercial supplier; whereby one part is applied to the base textile substrate via the above identified shear thinning process and the second part is applied via the film layer processing step. Upon cure, the two parts cross-link into a homogeneous elastomeric silicone polymer. The chemical and mechanical interlocking bond formed between the substrate/polymer layers is such that only cohesive failure in the polymer can be detected upon peel tests. This means that the adhesive strength of the substrate/polymer layers is greater than the cohesive strength of the polymer being applied in the film layer processing step. This amazing result was unexpected and has occurred for a variety of polymer compositions.
Various other and further features, embodiments, and the like which are associated with the present invention will become apparent and better understood to those skilled in the art from the present description considered in conjunction with the accompanying drawings wherein presently preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings and the associated accompanying portions of this specification are provided for purposes of illustration and description only, and are not intended as limitations on the invention.